Benthic metabolism and nutrient regeneration on the continental shelf of Eastern Massachusetts, USA
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Benthic metabolism and nutrient cycling were examined in depositional sediments of Broad Sound, Massachusetts Bay, Stellwagen Basin and Cape Cod Bay between 1990 and 1994 in water 16 to 76 in deep. Bottom water nitrate and dissolved oxygen concentrations were typically <10 muM and >70% of saturation. Sediment organic content was uniformly low at all sites ranging from 1.3 to 2.1 % carbon and 0.1 to 0.3 % nitrogen. Sediment chlorophyll a and phaeopigment concentrations averaged ca 2 and 15 ug cm(-3) respectively. Porewater nutrients and sediment-to-water fluxes of O-2, dissolved inorganic carbon (DIC), NH4+, NO2- + NO3-, urea, PO43-, N2O and dissolved silicon (DSi), were measured in cores from 4 stations, 4 to 6 times a year, for 2 yr, in order to examine seasonal and annual patterns. An additional 8 sites were examined over several years in late summer in order to examine spatial patterns. Average benthic community respiration ranged from 10.6 to 14.3 mmol O-2 m(-2) d(-1) for 12.1 to 29.8 mmol C m(-2-)d(-1). Within regions, respiration was highest in Broad Sound, followed by Massachusetts Bay, Cape Cod Bay and Stellwagen Basin. Sediments were sources of inorganic N. Annual average dissolved inorganic nitrogen (DIN) fluxes ranged from 0.5 to 1.8 mmol N m(-2) d(-1). For all stations, the flux of DIN was dominated by NH4+. The importance of NO3- fluxes relative to NH4+ increased with increasing depth. For example, in late summer the relative importance of NO3- fluxes increased from 0 to 44 % of total DIN flux between Broad Sound (<24 m) and Stellwagen Basin (75 m). Fluxes of N2O and urea were always extremely small (<0.1 % of DIN). Seasonal cycles, which in general tracked bottom-water temperature and phytoplankton sedimentation, were very strong for respiration (O-2 and DIC) but not for fluxes of NO3-, urea, N2O, and PO43-. Annual variation in benthic respiration was ca 25 %, similar to that for primary production. Inorganic nutrient fluxes often varied in excess of 100 % between years. Sediment mixing or porewater advection is thought to contribute to high nutrient flux variability. C, N, and P flux stoichiometry deviated substantially from the Redfield ratios, suggesting strong phosphorus retention by sediments and large nitrogen removal via denitrification. DSi fluxes were relatively high, which we attribute to focusing of biogenic material in these depositional sediments. Denitrification exceeded total DIN flux, averaging 1.9 and 1.3 mmol N m(-2) d(-1) in Massachusetts Bay and Stellwagen Basin respectively. The percentage of remineralized N that was denitrified increased with increasing water depth (from 60 to 72 % for 35 and 75 m water). Denitrification was supported almost exclusively by coupled nitrification and denitrification. The benthic community decomposed 12 to 22 % of overlying water primary production but only provided 3 to 8 % of the phytoplankton inorganic N requirement.